Water heaters, furnaces, boilers, fireplaces, and other combustible fuel-fired equipment or appliances generally include valving systems to connect and control the flow of gas between an external supply and their respective burners. These valving systems typically include normally closed, solenoid-controlled pneumatic valves so that the external supply of gas, which may be delivered to the valving systems via pipes connected to a gas supplier, a locally situated tank and/or other delivery system, only flows when called for. In other words, gas does not flow unfettered in normal operation.
Without such valving systems, not only would freely venting the gas supply be an extreme waste of a resource, but also, when vented in a closed or semi-closed environment, the gas volume in the locale of the gas-fired equipment might build. The build up of gas may create a potentially dangerous situation. For example, an explosion could result with a given amount of gas build-up and an ignition source. Over time, fairly sophisticated valving and valve-control systems have been developed to safely supply gas to the gas-fired equipment. Such systems, for example, may include direct-ignition combustion systems.
To limit inadvertent gas flow, some direct-ignition combustion systems employ a series of redundantly plumbed solenoid-controlled pneumatic valves. For gas to flow, all of the valves need to be open. Generally, to control the opening and closing of the solenoid valves, direct-ignition combustion systems typically employ electronic drive mechanisms, which have a plurality of electronic components. In operation, these electronic drive mechanisms energize and de-energize the solenoid coil of the solenoid-controlled pneumatic valves, which in turn causes the mechanical portion of the valves to open and close. And when all valves are mechanically open, gas flows.
Unfortunately, however, the electronic drive mechanisms and the electronic components can fail. When one or more components of these mechanisms fail, valves can be left open and cause gas to flow, which can cause an unsafe condition. To prevent this from happening, fail-safe electronic drives are employed. These fail-safe drives can prevent inadvertent flow through the redundantly plumbed valves when critical components in the drive systems fail. When the components of these systems fail, it would be desirable to determine which of the drive components have failed.
In typical fail-safe systems, two solenoid-controlled pneumatic valves are used. In these systems, the fail-safe mechanisms (i) control only one valve (and the other valve is controlled by another mechanism), (ii) employ separate mechanisms to control each of two valves, or (iii) use energy transferred from control of one valve to control the second valve (i.e., the first valve must be on for the second valve to turn on). As such, the fail-safe systems either lack the ability to independently control each of the valves single handedly or require a plurality of fail-safe circuits to independently control each of the valves.
Thus, it would be desirable to have a single, low-cost, fail-safe mechanism (e.g., fail safe circuitry) to (i) independently control each of the valves during normal operation, (ii) independently control each valve during testing, and (iii) diagnose and isolate faulty circuitry when one or more of the fail-safe mechanism and/or components thereof fails.